1. Field of the Invention
The invention relates to an apparatus and a method for forming and deploying a fully or partially immersed hollow concrete column designed for but not limited to providing a pipe for bringing cold water from the depths, for conducting sewage offshore, for building underwater tunnels, for supporting a structure above the surface, or for providing access to an undersea habitat or a subsea chamber which houses oil well equipment at atmospheric pressure.
2. Prior Art
______________________________________ References: ______________________________________ 3,249,664 5/66 Georgii 264/88 3,652,755 3/72 Iorns et al. 264/219 3,928,104 12/75 Luckett et al. 156/171 4,030,864 6/77 Einstabland et al. 425/65 4,443,131 4/84 Olsen 405/203 4,664,556 5/87 Dixon 405/223 ______________________________________
Large diameter concrete pipe has been considered unsuitable for use as the cold water pipe in Ocean Thermal Energy Conversion (OTEC) systems because of weight but has been widely used for outfall sewers. Pipe was fabricated on shore by conventional methods in segments whose size was limited by the capacity of the lifting and transporting equipment available. These segments were then transported to the construction site and assembled by various means to form an outfall sewer. Another method used horizontal floating forms which were periodically opened as the concrete hardened and then repositioned, but shifting of the forms was comparatively complicated, time-consuming and difficult to keep watertight.
It is also known to build caissons by pouring concrete between double forms and lowering the caisson as the pouring proceeds. The weight of the caisson is transferred to its floating support through the upper, last-poured portion of the caisson and its form, so that method can be used only for caissons having a comparatively short vertical length. The method is sensitive to wave motions which cause relative movements between the form and the upper part of the caisson which has not yet completely set.
U.S. Pat. No. 3,928,104 discloses a method and apparatus for manufacturing concrete pipes by depositing a layer of green concrete (i.e., concrete which has not set) on a mandrel, winding helically a flexible, binding member to consolidate and retain the concrete on the mandrel and winding wire reinforcing with substantial tension to prestress the pipe. The flexible binding member prevents the wire reinforcing from cutting into the green concrete and is an essential element in the method. By contrast, the present invention applies green concrete to the form by spraying with sufficient velocity to cause the concrete to adhere to the form and does not require a flexible binding to hold or protect the initial layer of concrete. Reinforcing wire needs to be applied with only enough tension to keep it in place.
It is also known to prestress concrete tanks by winding wire around the exterior and tensioning with special equipment. Prestressing is commonly used to reduce weight and control cracking in concrete. The present invention can use laminated ferrocement as disclosed in the inventor's prior U.S. Pat. No. 3,652,755 to achieve the same result in a thinner wall.
In norway, hollow monolithic thick-walled concrete columns for supporting oil rigs have been built on a floatable platform in a graving dock by placing concrete between sliding inner and outer forms, known in the trade as slipforms, which are raised by mechanical means as construction progresses.
As offshore structures became larger and heavier, inclined columns were needed to spread the heavy load over a broad base. U.S. Pat. No. 4,030,864 describes an apparatus for slipforming including free-standing hollow columns of concrete, but the degree of inclination and height of such columns is severely restricted without an elaborate supporting framework. U.S. Pat. No. 4,443,131 describes a further method in which columns are cast in a vertical position with conventional slipforms, then tilted toward each other and permanently connected together to form a support for the superstructure. The present invention eliminates the need for the mechanical jacks, jigs, and attachments required to raise slipforms yet also permits inclining columns to any angle without an elaborate supporting structure.
U.S. Pat. No. 3,249,664 discloses a method in which a comparatively short end section of the desired column is built first, which one end closed, then floated on the water with its open end projecting above the surface. Conventional forms or slipforms are mounted on the exposed rim and the entire weight of the concrete and formwork is supported by the upward thrust of the surrounding water. Since the form is attached to the column, no relative movement can occur between the concrete and the form due to wave action. As new concrete is added and gains strength, water is introduced into the cavity of the column to keep the forms at working level. When completed, the upper end may be closed and the water pumped out to bring the column into a horizontal position for towing. When the concrete column must have open ends, e.g., for tunnel sections, the first fabricated end section is made removable and can be used to start other sections. The present invention does not require an end piece to start production.
Suitable fabricating sites for the systems described above were often at a considerable distance from the installed site, resulting in high transportation costs. Further, the assembly and deployment of the final structure involved complicated equipment and risk, especially at depths below the reach of human divers.
The concept of applying successive layers of mortar or concrete to form a marine structure was disclosed by this inventor and his associate in U.S. Pat. No. 3,652,755, but was limited to the interior surface of a discrete female mold, whereas the present invention applies layers of concrete to either the interior of the exterior surface of a slipform.
Said patent also required that the reinforcing be forcibly pressed into the concrete while still in the plastic state by means of a special tool, whereas the present invention permits construction of both a reinforced and an unreinforced column by shotcrete methods without special tooling other than the apparatus hereinafter described.